M-256 was a Project 615 (NATO

NATO reporting name

NATO reporting names are classified code names for military equipment of the Eastern Bloc...

: "Quebec-class") short-range, diesel

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

 attack submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 of the Soviet Navy

Soviet Navy

The Soviet Navy was the naval arm of the Soviet Armed Forces. Often referred to as the Red Fleet, the Soviet Navy would have played an instrumental role in a Warsaw Pact war with NATO, where it would have attempted to prevent naval convoys from bringing reinforcements across the Atlantic Ocean...

. She was commissioned

Ship commissioning

Ship commissioning is the act or ceremony of placing a ship in active service, and may be regarded as a particular application of the general concepts and practices of project commissioning. The term is most commonly applied to the placing of a warship in active duty with its country's military...

 into the Baltic Fleet

Baltic Fleet

The Twice Red Banner Baltic Fleet - is the Russian Navy's presence in the Baltic Sea. In previous historical periods, it has been part of the navy of Imperial Russia and later the Soviet Union. The Fleet gained the 'Twice Red Banner' appellation during the Soviet period, indicating two awards of...

.

Design

Project 615 submarines were fitted with two regular diesel engines and a third, closed-cycle diesel engine, which used liquid oxygen

Liquid oxygen

Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...

 (LOX) to provide air-independent propulsion

Air-independent propulsion

Air-independent propulsion is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion...

 while the submarine was submerged. This system produced remarkable submerged speed and range, and greatly increased the hazard of a fire. Project 615 submarines were referred to by their crews as "cigarette lighters."

Sinking

On 26 September 1957, while operating in gale conditions

Beaufort scale

The Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort Wind Force Scale.-History:...

 in the Gulf of Finland

Gulf of Finland

The Gulf of Finland is the easternmost arm of the Baltic Sea. It extends between Finland and Estonia all the way to Saint Petersburg in Russia, where the river Neva drains into it. Other major cities around the gulf include Helsinki and Tallinn...

 of the Baltic Sea

Baltic Sea

The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...

, one of M-256’s diesel engine

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s exploded. Fire immediately engulfed the diesel compartment, and soon spread to the next compartment. The boat surfaced and because of the likelihood of further explosions her crew evacuated onto her weather deck. None of the four ships keeping station nearby were able to take her under tow or evacuate her crew because of the gale conditions. About four hours after the beginning of the fire the boat suddenly lost longitudinal stability, took on a steep down-bubble, and sank. Of the 35 men on the boat's deck, only seven were rescued.